THE AFFERENT PATHS OF SENSORY IMPULSES 871 



which are obliterated, and touch, heat, cold and pain which remain 

 normal. 



Lesions of more limited distribution may produce dissociations of a 

 more simple character. Because the fibers conducting the impulses for 

 pain, heat, and cold are grouped together into separate tracts for each 

 quality of sensation a limited lesion may destroy one of these qualities 

 without affecting the others. The close association of the three groups, 

 however, makes it probable that a lesion affecting one will affect them 

 all, so that in the majority of cases heat, cold and pain will all be affected 

 together. When dissociation occurs as the result of spinal lesions, cu- 

 taneous and deep sensibility of any given quality suffers alike, and as a 

 result these dissociations have a distinctly different character from those 

 due to peripheral nerve lesions. 



The loss of sensation which results from interruption of the afferent 

 tracts in the cord affects parts which may be remote from the lesion. 

 But if the destruction involves only the gray matter of the cord, it will 

 not disturb those impulses which pass through either the crossed or 

 uncrossed tracts of the cord at the level of the lesion, but only those 

 whose paths entered the gray matter and cross the cord. As a result the 

 sensation of heat, cold, and pain may be absent from a segmental area on 

 the side of the body corresponding to the lesion, or if the destruction is 

 more extensive, these sensations may be absent from a segmental band 

 completely encircling both sides of the body, without any disturbance 

 affecting the sensation of the lower segments, impulses from which have 

 crossed the cord at a lower level and pass the lesion in the tracts of the 

 lateral column. Segmental disturbances of this type are distinguished 

 as the local effects of an intermedullary lesion. If the lesion is exten- 

 sive so that both white and gray matter is destroyed on one side of the 

 cord, a combination of the local and remote effects occur, with the re- 

 sult that pain and the temperature senses are lost over the entire half of 

 the body opposite to and below the lesion and also over a local segment 

 at the level of, and on the same side as, the lesion. 



Afferent Paths in the Brain Stem 



On reaching the medulla all impulses producing sensations of pain, 

 heat, cold, and touch (including one dimensional localization) have 

 crossed the cord. . They continue without interruption through the brain 

 stem in the tractus spinothalamicus lateralis to terminate in the nuclei 

 of the optic thalamus. In their course they are joined by impulses from 

 the cranial nerves, which have also crossed to the side of the brain stem 

 opposite to that from which they have entered. Impulses which give rise 

 to the recognition of posture, passive movement, and two dimensional 

 localization reach the medulla 011 the side from which they have origi- 



